As is known, a radio receiver, especially in a multimedia system of an automotive vehicle, is able to receive a radio signal, in particular an AM radio signal, AM being the abbreviation of “Amplitude Modulation”.
Such an AM radio signal, received in modulated form by a radio receiver, is subjected to various sensors and to a suitable filtering adapted so that the corresponding demodulated radio signal can be rendered under good conditions, especially in the cabin of an automotive vehicle.
The person skilled in the art knows the operating principle of an AM, that is to say amplitude-modulated, radio signal received by a suitably adapted radio receiver, with a view to being demodulated and then rendered to listeners.
Thus, an AM radio signal comprises a carrier p(t) of the signal, of frequency fp, satisfying the equation:p(t)=A·sin(2·π·fp)
and a message m(t) to be transmitted satisfying the equation:m(t)=M·cos(2·π·fm·t+φ)
The corresponding modulated signal y(t) then satisfies the equation:
                    ⁢                            y          ⁡                      (            t            )                          =                              [                          1              +                              m                ⁡                                  (                  t                  )                                                      ]                    *                      p            ⁡                          (              t              )                                          ,                          ⁢                          ⁢              i        .        e        .                  :                                y      ⁡              (        t        )              =                  A        ·                  sin          ⁡                      (                          2              ·              π              ·                              f                p                            ·              t                        )                              +                                    A            .            M                    2                ·                  [                                    sin              ⁡                              (                                                      2                    ·                    π                    ·                                          (                                                                        f                          p                                                +                                                  f                          m                                                                    )                                                        +                  φ                                )                                      +                          sin              ⁡                              (                                                      2                    ·                    π                    ·                                          (                                                                        f                          p                                                -                                                  f                          m                                                                    )                                                        +                  φ                                )                                              ]                    
From a spectral point of view, starting from a message m(t) composed of several frequencies, the amplitude of the Fourier transform FFT of the modulated signal y(t) containing the message m(t), as a function of frequency F, is represented in FIG. 1.
To demodulate the AM radio signal, the latter is converted to baseband, in such a way that the message m(t) is centered around 0 Hz, as represented in FIG. 2.
A known problem resides in the fact that the AM radio signal received may be noisy or disrupted by an adjacent radio signal, making its rendition unsatisfactory. Therefore, it is known to filter the demodulated radio signal, before its rendition, with a view to eliminating therefrom or to limiting the noise and the disruptive interference due to the adjacent signals.
However, the filtering performed in the demodulated signal is applied by definition to the message, that is to say to the relevant data of the received radio signal, such as the audio signal to be rendered. The filtering performed at this level of the audio chain of the multimedia system, that is to say after demodulation of the radio signal received, therefore presents the drawback of inducing an adulteration of the data contained in said radio signal.